1. Technical Field
The present invention relates to a heating control circuit, and more particularly, to a process and a circuit for controlling the temperature of an electronic system to a preset temperature by sensing a disconnection of a temperature sensor located within the electronic system having a heating element such as a heater.
2. Background Art
Generally, electronic systems such as, by way of example, laser beam printers, laser array printers, copiers, photomultilithographic duplicators, facsimile telecommunication units, and other office machines, perform a predetermined operation using an input of a supply voltage, and execute a corresponding operation in response to an exterior command. Typically, these electronic systems have a heater for preparing the operation of the system, and the heater maintains the temperature of a fusing unit to a preset temperature in order to fuse a developing material, called toner, onto copying or printing paper during a copying or printing operation. As is well known in the art, if the temperature of the fusing unit positioned within the system has been not preheated to the preset temperature, the toner will not be fused onto the copying or printing paper. Therefore, office machines with a heater must both heat the fusing unit by appropriately driving the heater during a stand-by mode and lower the temperature of the fusing unit by controlling the heater to not drive the heater whenever the fusing unit is in an overheated state.
In such laser beam printers or copiers, the temperature of the fusing unit is about 150.degree. C. during a stand-by state and about 180.degree. C. during a printing state. The temperature of the fusing unit is controlled by a heater control circuit for controlling the temperature of the heater to a temperature within the range of 150.degree.-180.degree. C., in response to the output of a temperature sensor for sensing the temperature of the fusing unit. Therefore, office machines having a heater necessarily include a temperature sensor for sensing the temperature of the interior of the system in order to appropriately control the temperature. Moreover, the heater control circuit controls the temperature of the heater to a preset temperature in response to the output of the temperature sensor, in order to adjust the temperature of the fusing unit.
The system including the heater control circuit for controlling the temperature of the fusing unit in response to the output of the temperature sensor has a circuit for checking the temperature sensor in order to prevent the fusing unit from overheating. That is, if the temperature sensor is disconnected or separated from a connector, an error in detection of the temperature of the temperature sensor will occur; if the heater control circuit continuously the heater energizes, the temperature of the fusing unit will overheat, and as a result, a fire may occur. Therefore, a system with the heater should have a heating control circuit for preventing the fusing unit from overheating.
One design of a heating control circuit for preventing the fusing unit from overheating uses a thermistor to sense the temperature of the heater and generates a temperature detecting voltage corresponding thereto with a preset disconnection detecting voltage circuit and produces a disconnection signal when the disconnection detecting voltage is greater than the temperature detecting voltage.
Such a circuit shown can not easily detect the disconnection of the thermistor, and as a result, it is difficult to accurately control the temperature of the fusing unit. That is, when the thermistor is disconnected or the ambient temperature is very low, for example, about 5.degree. C., the level of the temperature detecting voltage is nearly the same. In more detail, if the thermistor is displaced, dislodged or disconnected from proximity to the fusing unit, or if the ambient temperature is very low, the temperature detecting voltage is very high, and thus, it is difficult to set the disconnection detecting voltage so as to accommodate a range of values of the temperature detecting voltage that corresponds to a very low ambient temperature.